The present invention is directed toward the preparation of latex films having improved adhesion to polymeric substrates. The adhesion is provided through the formation of covalent bonds between the latex film formed from an application of an aqueous dispersion of polymeric particles and the polymeric surface. The bond formed is resistant to hydrolyzing and therefore is resistant to delamination under high humidity conditions.
Generally, it is very desirable to coat plastic or other polymeric surfaces with a latex composition. Unfortunately, latex films do not adhere well to the polymeric surfaces. This is in part due to the low surface energy, i.e., non-wettability, and nonpolar or hydrophobic characteristics of the plastic or polymeric surface and the aqueous nature of latex compositions. Even further, once an attempt to coat a plastic or polymeric surface is successful, usually the latex film so formed is very susceptible to delamination in the presence of water or humidity.
Thus, the formation of a bonding mechanism between the plastic or polymeric surface and the latex film is desirable. One such attempt is to employ a corona treatment to the polymeric surface to be coated which places a polar group on the surface and facilitates the application of the latex composition. However, this is not always effective in adhering the dried latex film to the treated polymeric surface.
Another attempt to improve adhesion is to roughen the substrate surface. Although this may improve the apparant contact angle and assist in the formation of a latex film, the film is still susceptible to delamination under various environmental conditions.
One area where it is very desirable to coat a plastic surface with a latex is in the preparation of plastic substrates with polyvinylidene chloride latex films to improve their barrier resistance. Polyvinylidene chloride latex films or, more commonly, saran latexes are especially effective to reduce the permeability of gases such as oxygen, carbon dioxide and vapors such as water through the plastic. However, problems exist in applying an adherent latex film to a plastic surface as mentioned earlier. It is especially difficult to maintain an adherent film under high humidity conditions.
Adhesion of the latex coating to the plastic substrate is a critical property requirement especially in humid environments. For example, where a polyvinylidene chloride latex coated plastic is employed to package beverages or food with substantial water content.